This application is a continuation application of PCT/DE02/02418 filed Jul. 3, 2002 which claims priority of German Application DE 101 31 887.1 filed Jul. 4, 2001, now abandoned. Priority is claimed under 35 USC 119(a)-(d) or (f) or 365(b) or (c) and under 35 USC 120.
The invention relates to a partition for use in the production of one or more multilayers or a multilayer pressed packet. The invention furthermore relates to a method for producing a corresponding partition, and a method for producing a multilayer pressed packet.
Various designs of partitions for use in the production of a multilayer pressed packet, or various methods for producing a multilayer pressed packet, that is for the pressing of multilayer printed circuit boards, are known in the state of the art. The pressing of the multilayers (printed circuit boards) generally takes place in multi-plate or vacuum presses, whereby the individual layers of the multilayer are correspondingly pressed together with one another at a temperature of ca. 180θC.
In general to accomplish this, first a pressed packet body is created within a suitable press. In this, several multilayers are placed one on top of another between two press plates, the corresponding pressing tools and press pads, and are separated from one another by corresponding partitions or pressing sheets. Each individual multilayer—ordinarily—is constructed as a multilayer. Namely it comprises several sheet molding compound layers, separated from one another by laminate layers. Between the individual laminate layers or sheet molding compound layers, corresponding copper foils are provided for the realization of corresponding wiring paths. (The individual layers may also be referred to as “plies”.)
Various problems now exist in the production of the multilayer itself. On one hand, due to the thermal expansion of the partitions, an unfavorable displacement/movement of the individual layers of the multilayer (the sheet molding compound or the laminate layers and/or also the copper foils placed between them) can occur. On the other hand, it is also important that with the help of the partitions, the pressure within the multilayer pressed packet also be evenly transferred to the individual sheet molding compound or laminate layers of the multilayer pressed packet, in order to produce an optimal connection in a multilayer and to prevent any slipping to one side of the individual layers as far as possible.
According to DE 38 44 498 A1, a deformation of multilayer circuits that are pressed together and an uneven copper surface can be prevented by using vacuum and isostatic pressing techniques in the pressing process. In this process, additional plates are supported in a floating manner and are thermally insulated, to keep the press plates cool during the heating of the multilayer press packet. The disadvantage here is that during the heating process epoxy resin that is exuded can bond the edges of the multilayer circuit together.
According to DE 35 07 568 C2, a slippage of the sheet molding compound layers to one side can be prevented by means of an anti-slip device. This can, however, prevent a possible leakage of the sheet-molding compound to the sides of the packet only if the copper foils are selected somewhat larger than the other layers, so that the sheet-molding compound can run off onto them. At any rate, this method requires a longer heating period, and the heat penetration is substantially less even than when, for example, aluminum sheets are used as the partitions.
According to DE 41 16 543 A1, a partition made of high-grade steel is used, which has a specific heat expansion coefficient that approximates the expansion coefficient of the copper. In this manner, surface tension is prevented to the greatest possible extent, but the time required to heat this high-grade steel sheet is correspondingly long.
For this reason, in the past people have gone to using predominantly aluminum sheets of certain alloys as partitions, which can better and more evenly conduct the heat. In all the methods named above, the copper foil and the corresponding partition must be fitted together with the other layers by hand (manually) by specially trained personnel, in an assembly room provided for this purpose. One problem with this, among others, is that the often very thin and thus sensitive copper foils become rapidly crushed in this process.
The above-described partitions used or methods for producing multilayer pressed packets are not yet optimal for the next generation of printed circuit boards (multilayers), especially for UMTS [universal mobile telecommunication system] technology. The wiring paths are becoming narrower and narrower, while at the same time more and more must be accommodated on decreasing surface areas, especially with the help of “HDI-high density interconnect technology”. Because the copper foil to be used is becoming thinner and thinner, with thicknesses of <12 Πm or even 5 Πm already being mentioned, this also increases the danger that in the pressing of the multilayers, the wiring paths of the inner layers could press through the outer copper foils. When this effect occurs it is referred to as a so-called “image transfer”. The problem with this is an uneven abrasion of the thickness during etching, and corresponding inaccuracies with boring. And especially, these extremely thin outer copper foils can no longer be manually applied. They must therefore also be produced as a composite using a partition or pressing sheet, as is described in DE 198 31 461 C1.
Thus in DE 198 31 461 C1 a method for the partial joining of copper foils of any type and thickness using an aluminum pressing sheet (partition) of any alloy and thickness is described, in which a composite of aluminum pressing sheet and copper foils is produced, whereby this composite is used in the production of corresponding multilayer pressed packets. The problem with this, however, is that a partition or pressing sheet made of aluminum or a suitable aluminum alloy that will have sufficient hardness to prevent the above-described “image transfer” cannot be produced. The aluminum alloys that have been used up to now as the above-named aluminum pressing sheet currently possess a strength of Rm 400 Mpa. The pressing of the multilayer takes place at a temperature of ca. 180θC over a period of ca. 90 minutes. At this temperature, the strength of the aluminum pressing sheet drops to ca. Rm 360 Mpa. As a result, the use of known partitions presents problems for the next generation of printed circuit boards.
It is thus the object of the invention, based upon the initially mentioned partitions, or the above-described methods, to create and refine a partition for use in the production of a multilayer or a multilayer pressed packet, or the method for producing a multilayer or a multilayer pressed packet, such that the so-called “image transfer” is prevented in a cost-effective manner for the partition.